For the purpose of the present invention, the term “underwater pipeline” is generally intended to mean a pipeline laid on the bed of a body of even deep water to conduct gas and/or fluid, in particular oil.
An underwater pipeline of the type described covers distances of hundreds of kilometers, is composed of pipes joined to one another on a laying vessel, is launched into the water off the laying vessel, and is laid on the bed of the body of water as it is assembled.
Each pipe is normally of 12-meter unit length, has a relatively large diameter ranging between 0.2 and 1.5 meters, and comprises a steel cylinder; a protective covering of polymer material contacting the steel cylinder to protect it from corrosion; and sometimes a heavy covering of concrete or Gunite contacting the protective covering to weigh the pipe down.
The pipes are joined at on-land installations into pipes of multiple unit length, and on laying vessels on which the pipes of unit or multiple unit length are joined to form the pipeline, which is then launched off the laying vessel onto the bed of the body of water.
The pipeline is assembled and launched off the laying vessel using one of two methods, depending on the depth of the body of water.
In a first method, the pipeline is formed on a launch tower comprising an assembly station, and is launched substantially vertically so that it assumes a J-shaped configuration between the laying vessel and the bed of the body of water. This method is particularly suitable for laying pipelines in very deep water.
In the second method, the pipeline is formed on a substantially horizontal assembly line, and is launched off a lay ramp which, in the work configuration, serves to guide and support the pipeline along a curved path having a first portion above water, and a second portion below water. Pipelines laid using this method assume an S-shaped configuration between the laying vessel and the bed of the body of water.
Close to coastlines and in shallow water, pipelines are normally buried in the bed to protect them against hydrodynamic stress, changes in temperature, and damage by foreign bodies.
Burying underwater pipelines in the bed is a common practice in shallow water, but is difficult to do and economically unfeasible in deep water. As a result, pipelines simply laid on the bed in deep water are exposed to blunt objects, such as inadvertently towed anchors, which literally “plough” the bed and may damage the pipeline, even to the point of ripping it apart. Incidents of this sort are relatively rare, but the damage caused by them is enormous, both in terms of pollution and the fact that many countries depend almost entirely on such pipelines for their energy supply.
When they do occur, therefore, steps must be taken immediately to repair, seal, and restore the mechanical characteristics of the pipeline.
Various methods of doing this have been proposed, some of which provide for carrying out all the repair work in the body of water, and others for performing part of the repair work above water, and part in the body of water.
A repair method described in U.S. Pat. No. 5,437,517 comprises the steps of cutting out the damaged length of pipeline to form a first and second pipeline portion having a first and second end respectively; joining a first coupling head to the first pipeline portion in the body of water; joining a second coupling head to the second pipeline portion in the body of water; laying a telescopic sleeve, having a third and fourth coupling head, in the body of water, between the first and second coupling head; aligning the first pipeline portion, the second pipeline portion, and the telescopic sleeve in the body of water; adjusting the length of the telescopic sleeve in the body of water; and joining the telescopic sleeve to the first and second coupling head in the body of water. The telescopic sleeve, which comprises two sliding tubes, is then locked into position. All the above operations are performed in the body of water using underwater equipment controlled by ROVs (Remote Operated Vehicles) connected by cable (umbilical) to a laying vessel.
One of the most critical parts of the above method is aligning the first and second pipeline portion and the telescopic sleeve; and the deeper the water is, the more difficult alignment becomes. Adjusting the length of the telescopic sleeve is also a delicate operation, by having to be performed without moving the sleeve out of line with respect to the first and second coupling head.
An underwater pipeline repair method described in U.S. Pat. No. 4,304,505 comprises the steps of cutting out the damaged length of pipeline to form a first and second pipeline portion having a first and second end respectively; raising the first end onto a laying vessel; joining a first coupling head to the first pipeline portion; laying the first end and the first coupling head onto the bed of the body of water; raising the second end onto the laying vessel; joining a second coupling head to the second pipeline portion; laying the second end and the second coupling head onto the bed; laying a pipe section, having a third and fourth coupling head, in the body of water, between the first and second coupling head; aligning the first pipeline portion, the second pipeline portion, and the pipe section in the body of water; and joining the pipe section to the first and second coupling head in the body of water.
This method too involves a critical aligning stage, which is vital for achieving connections capable of restoring the mechanical characteristics and fluidtight sealing between the pipe section and the first and second pipeline portion.
Accurately aligning the first and second pipeline portion and the telescopic sleeve or pipe section calls for the use of an extremely bulky, heavy aligning device designed to engage and align the first and second pipeline portion and the telescopic sleeve (or pipe section, in the case of the method described in U.S. Pat. No. 4,304,505). The aligning device, however, can only operate on a relatively flat bed capable of stably supporting the weight of the device.